"I said, 'Why don't we just bet?'" Ben told us. "I've read enough of Felix's stuff to know that would be irresistible to him."

"He's right about that," Felix said. "Being challenged by Ben Horowitz is kind of the high point of my career. So I immediately said yes."

When we heard about the challenge, we invited Felix and Ben to come on Planet Money to hash out the details of the bet. Basically, we offered to be their bookie. Fortunately for us, they accepted. (Listen to the show.)

If you can't wait to see what the bet is, scroll down to the bottom of this post; the details of the bet are described in bold.

The Case For And Against Bitcoin

Before we got to the details of the bet, we asked Ben and Felix to make their cases. Ben went first.

Most of the coverage of bitcoin has focused on its skyrocketing value — from a few cents per bitcoin in 2010 to over $1,000 per bitcoin last year.

But Ben's belief in bitcoin isn't based on how much it costs to buy a bitcoin. It's based on the belief that bitcoin is a major technological advance that will make it cheaper and easier for people to buy and sell stuff over the Internet.

Ben: It's a real computer science breakthrough. This is a problem that we've been trying to solve in computer science since the early '80s, which is: How do you prevent the double-spending problem?

In other words: It's very easy to make copies of digital things (think of all the piracy of digital music and movies). But for digital money to work, it has to be impossible to make copies. Otherwise, counterfeit money would be rampant and the system would collapse. And bitcoin has solved that problem.

And, Ben says, the way we buy stuff online now — basically, using credit and debit cards — is a problem for a lot of businesses that sell stuff online. For one thing, the businesses have to pay credit card companies a fee of around 2.5 percent of everything they sell. And for companies with tight profit margins, that fee is a big deal.

Also, Ben says, credit card companies reject some of perfectly legitimate customers because they get flagged as potential frauds.

Ben: As a merchant, you end up rejecting approximately 8 to 10 percent of your good customers due to potential fraud. Netflix, I think, only accepts customers from 40 countries because of this credit-card problem. And merchants in, like, Kazakhstan, they're just done — there's no way they can be online.

So that's Ben's case. And, to some extent, Felix actually agrees with him: he says the the current way of paying for stuff online is a big problem. But, Felix says, bitcoin has its own fundamental problems.

One of the biggest problems Felix cites with bitcoin is somewhat counterintuitive: The fact that bitcoins have become so valuable so fast means no one is going to want to spend them to buy stuff. Instead, people will just buy them and hold onto them.

Felix: I'm skeptical about bitcoin in particular partly because it is a store of speculative value. It's a place where speculators like the Winkelvoss twins will come in and buy up lots of bitoins for no reason other than they think they can find a greater fool to sell the bitcoins to tomorrow.

In fact, Felix points out, the way the bitcoin system was designed — the way the computer the code was written — only a finite number of bitcoins can ever exist. And that fact alone means they're likely to get more and more valuable over time. In other words, deflation is built into the system.

Felix: And because bitcoin offers the potential for so much profit just by sitting on it — the longer you don't spend it, the more you benefit — then everyone has an incentive to not spend their bitcoins. And you can't have an have effective payments mechanism if you're not spending the currency.

Ben thinks the rise in bitcoin price will slow significantly.

Negotiating The Bet

First thing: What's the duration of the bet?

Ben: I think it makes sense in the five- to seven-year time frame. It is a big thing to upgrade the the payment structure of the Internet if not the world. ...

Felix: There's no way that bitcoin is going to be a common payment mechanism in five years' time. It probably will not even exist. It's just going to be a vague memory.

They agree to five years.

Next: What are the terms of the bet?

Felix and Ben didn't want to bet on the price of a bitcoin in dollars. That could just be the result of people speculating. They wanted to bet on whether people were actually using bitcoins to buy stuff.

After some back and forth — should they pick a particular retailer? If they did, would we actually know what share of a retailer's business was in bitcoin? — we offered an alternative.

We offered to do a poll five years from now to ask them whether they used bitcoin to buy stuff. Ben and Felix negotiated the specifics, and came up with the following.

Here's the bet:

Five years from now, in January, 2019, we'll poll a representative sample of Americans. If 10 percent or more say they have used bitcoin to buy something in the past month, Ben wins. If it's fewer than 10 percent, Felix wins.

Last question: What are the stakes?

Betting a bitcoin was a no-win proposition for Felix. If he's right, the value of a bitcoin will be trivial in five years. If he's wrong, his cost could be astronomical.

For stakes, they settled on one of the first things you could buy with bitcoin: a pair of alpaca socks.

That gave Felix one last chance to make his case.

Felix: Part of the reason I am going to win this bet is exactly that everyone who bought alpaca socks for bitcoin two years ago is kicking their warm feet right now saying, "I wish I hadn't bought those alpaca socks. I wish I'd held onto the bitcoin because now I'd be a millionaire."

Imagine you could call up a friend and say, "Meet me at the bar and tell me what's going on with the economy." Now imagine that's actually a fun evening. That's what we're going for at Planet Money. Want to know more? Check out our "about" page. Want to connect with the Planet Money team? Send us an email.